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Aug. 7, 1951 w.`c:.. scoTT LEACHING COPPER ORES Filed June 10, 1948 2Sheets-Sheet l r '1/9 Nom smaad Nollvmko 9433/ we en -w Aug. 7, 1951 w.G. scoTT 2,563,623

` LEACHING COPPER oREs Filed June lO, 1948 2 Sheets-Sheet 2 TIME oI=voxfnATIoN, wEEKs FERRIG IRON IN soLuTIoN; nAsI-IED cuRvE-, Tom. FERRIGIRON 80TH IN soLuTIoN a IN PREGIPITATE TIME oI= oxIDATIoN, wI-:EKs

SOLID CURVE ATTORNEYS Patented Aug. 7, 1951 UNITED STATES PATENT OFFICEApplication lune 10, 1948, Serial No. 32,150

A11 Claims. (Cl. 'i5-104) This invention relates to` leaching sulphidicMcopper ores, and is directed particularly to the provision of animproved method for leach-ing such ores in place. Such leachinginvolvespassing a dilute aqueous solution of sulphuric acid.;`

and ferrie `sulphate through the ore. The invention contemplatescontrolling the concentration of iron in the leach solution, and its pH,within limits found to .be critical (l) for avoiding plugging ofsolution channels in the ore and.: (2) to enable ferrous-.iron to bereoxidized economically by the air to the -ferric condition.

Leaching of copper ores in place is a procedure for recovering copperfrom ores containing it in worthwhile amounts, but that are off;`

satisfactory leaching agent. Iron -py-rites, in theyA icn presence ofwater and oxygen from the air., oxi' dizes quite readily to `ferroussulphate and `sulphuric acid,

and the ferrous sulphate then becomes oxidized to the ferrie form,

formed are effective reagents for dissolving both oxidized and sulphidecopper minerals.

On account of the quite rapid progress of the rst of the foregoingreactions, pyritic copper ores are commonly .known inthe leaching artasi acid-making ores. The production within the orebody itselfofsufcient sulphuric acid to prevent basic ferric ,sulphate `fromprecipitating in the solution channels and gradually plugging them up isan important factor contributing to, the success of leaching pyriticcopper ores in place. Further, since the ferrie sulphate itself is madewithin the orebody, there is no problem involving oxidation oa ferroussulphate solution to the ierric form before it is delivered to the`orebody.

In addition to the pyritic orebodies that have been leached in placesuccessfully, there are a considerable number of .known sulphidic butnonpyritic copper orebodies of substantial extent containing too littlecopper to be mined at a prot, but which are well-suited to leaching inplace. For example, in districts where low-grade porphyry copper oresare found, there are often large marginal portions of the `orebody whichbecome broken up in the course of mining the richer zones, but which donot contain enough copper to make extraction of the broken marginalmaterial worthwhile. Such ore is well adapted to leaching in place, butcannot be leached by the method that has been successful on pyriticores. The lack of pyrites in the ore makes it impossible to leach withwater alone, as no sulphuric acid or ferrie sulphate is formed in theorebody to dissolve the copper. Leaching in place with sulphuric acidalone results in extracting only such copper as is present in oxidizedminerals-it does not extract copper from sulphide minerals. If ferriesulphate is added to the leach solution to dissolve the sulphidic copperminerals, two problems present themselves that have not heretofore beensolved: (1) the problem of preventing the ferrie sulphate fromprecipitating as a basic sulphate in the solution channels within theorebody and gradually plugging them, as the sulphuric acid initiallyadded to the solution is consumed during leaching with consequentdecrease in the acidity of the solution, and (2) the problem ofeconomically providing an adequate supply of ferric sulphate inthe leachsolution.

The present invention provides an `effective solution to both of 'theseproblems. I have discovered that if the leach solution delivered to theore contains less than 10 grams per liter of iron, and preferably notmore than 8 grams per liter, and if suicient sulphuric acid is initiallypresent so that the solution after draining through the ore has a pH notgreater than about 2, no plugging of the solution channels in theorebody occurs in consequence of the precipitation of basic ferriesulphate. I have further discovered that in a solution containingsulphuric acid, ferrous sulphate (to which form the ferrie sulphate isreduced 'in leaching and in which form iron enters solution duringrecovery of the copper by cementation) can be oxidized to the ferrieform and used again in leaching by simple exposure of the 'solution tothe oxygen of the air, provided Vthe concentration of iron in theair-exposed solution is less than 25 grams per liter, and preferablyless than 15 grams per liter; 'and that if thepI-I of the solution thusexposed 'to the air is not'l'ess 'than about 2, excess `iron `there- 'inprecipitates as V'a 'basic ferrie sulphate, reducing the ironconcentration to an optimum value (less than grams per liter) forleaching.

Based on these discoveries, my invention provides for leaching sulphidiccopper ore in place by delivering a dilute aqueous solution of sulphuricacid and ferrie sulphate to the ore, and collecting the solution afterit has passed through the ore characterized in that the total amount ofiron present in the solution delivered to the ore is less than 10 gramsner liter, and the amount of sulphuric acid therein is such that the pHof the solution when it is collected after passing through the ore isnot greater than about 2. Further. after the copper has been extractedfrom the leach solution, the invention provides for making the solutionavailable for reuse, by

oxidizing its ferrous iron content to the ferric form and precipitatingexcess iron therefrom. Such oxidation is effected in accordance with theinvention bv introducing the solution into a body thereof that isexposed for at least several days to the oxygen of the air, the totalamount of iron in the solution being limited to less than 25 grams perliter (and preferably to less than grams perv liter) at Ythe time of itsintroduction into said Ybodv, and the amount of sulphuric acid in thel,solution then being such that its pH is not less than -about 2.Thereby the ferrous iron becomes oxidized and excess iron precipitatesas basic ferrie sulphate.

The invention is described in somewhat greater detail below inconiunction with the acompanying drawings, in which Fig. l showsschematically a preferred embodiment of the invention for leachingsulphidic but non-pyritic copper ore in place;

Fig. 2 is a series of charts (A to G) showing how variations intheconcentration of iron in a solution initially containing iron as ferroussulphate affect oxidationfthereof upon exposure tg air; 1 Y f Fig. 3 isa chart showing how the eiciency of the oxidation of a ferrous sulphatesolution exposed to the air varies with the totalamount of iron insolution; and l Fig. 4 is a series of charts (H to N) showing the effectof pH variations on the Way in which fer rous iron is oxidized andprecipitated upon ex- Yposure of a ferrous sulphate solution to air.

Referring rst to Fig. 1, the method of the invention involves deliveringferric sulphate solution from a storage pond ID by a pump II through'apipe I2 and sprays I3 to the surface I 4 of the ground overlying the orein place to be leached. The concentration of iron in this solutionisless than l0 grams per liter and preferably is less than 8 grams perliter. sulphuric The orebody Il to be leached is shown diagrammaticallyas having been broken and opened to the.penetration of leach solution bymining1 operations previously conducted from drifts I8 and raises I 9.The solution of ferric sulphate and sulphuric acid seeps down throughthe column of broken ore I1 into the raises and drifts,

collecting ultimately in one or more sumps in ,these mine workings. Inits passage through the A ore-[oxidized copper minerals are dissolved by`Athesulphuric acid present in the solution, and

-sulphide minerals, especially chalcocite, are dissolved .by the ferriesulphate (the Aferric sulphate4 thereby becoming reduced to ferroussulphate).

The amount of sulphuric acid added from the container I5 prior todelivery of the solution through the sprays I3 is suiiicient so thatafter the solution has percolated down through the ore and has collectedin the sump 20, its pH is not significantly greater than 2. If less thanthis amount of sulphuric acid is used, or if the iron in-solutionexceeds the limiting value of 10 grams per liter at the time of itsdelivery through the nozzles I3, basic ferric sulphate precipitates inamounts suiicient to plug up solution channels through the broken ore.Such an occurrence may lead to the loss of the orebody for leaching inplace, by making it impossible or uneconomically diflicult for the leachsolution to penetrate through it.

The copper-laden solution collecting in the sump 20 is delivered by apump 2l through a pipe 22 extending up through a shaft 23 to one or morecementation launders 24 containing scrap iron 25. Here the coppersulphate in the solution is precipitated as metallic cement copper,

which can be treated by conventional methods to produce metallic copperin marketable form. Precipitation of thecopper is accompanied bydissolution of an ecpuivalent amount of iron as ferrous sulphate. Somefurther quantity of ferrous sulphate also is introduced into thesolution as a result of direct chemical attack on the scrap iron by theresidual sulphuric acid. The ferrous sulphate thus entering solutionduring precipitation of the copper is in addition to the iron sulphate(most of which was reduced to the ferrous form during leaching) that wasoriginally present. In consequence, copper-free effluent solutionWithdrawn from the precipitation launder 24 contains most of the iron insolution in the ferrous form, and contains more iron in solution than isdesirable if the solution is to be reused in the leaching cycle. Theferrous iron is oxidized and the excess iron is precipitated bvdelivering the effluent solution through a stream or other conduit 26 tothe main body of solution in the pond I0. The pond shouldhave a fairlylargesurface-to-volume ratio (i. e. it should be rather shallow and ofbroad surface extent) to present as great an area of contact to.theoxvgen of the air as is reasonably possible; for it is by exposureof the solution in the pond to the oxygen of the air that oxidation andprecipitation are caused to occur.

It is necessary that the solution as it is withdrawn from thecementation launder 24 contains less than25 grams per liter of iron, andpreferably less than 15 grams per liter. If it contains more than 25grams per liter, virtually none of the ferrous iron Will be oxidized inany reasonable period of time, and if it contains more than 15 grams perliter, the extent of such oxidation will be decidedly incomplete.Furthermore, the effluent solution should have a pH not less thanl about2. OtherwiseI the undesirable excess of iron, above the maximum of 10grams per liter permissible for leaching, will not be precipitated. If,however, the iron'concentration and the pH of the solution are keptWithin the limits stated, mere exposure of the solution to the air inthe pond I0 for a period of several Weeks is suiicient to effectoxidation of the ferrous iron to the ferric form, and to bringaboutprecipitation of excess iron in the form of basic ferrie sulphate. Thepond should be large enough to contain at least several Weeks supply ofleach solution in order to give ample time for oxidation andprecipitation to takerplace.

If thel solution acidified` with sulphuric acid and delivered to the orethrough the sprays I3 contains some ferrous sulphate, it willy beatleast in part oxidized to the ferrie.- form by reaction with dissolvedoxygen from the air asit seeps down through the porous ground. Advantagecan be taken of this fact to minimize the volume of solution that mustbe held in the pond IU for oxidation, by diverting a portion of theeffluent from the ceinentation launder 24 by means of a pump 2l througha pond' by-pass pipe 28` directly to the pipe I2 through which solutionis delivered to the ore. The ferrous sulphatein this portion of theefiiuent then will be. largely oxidized in trickling down through the.ground, rather than in the pond, after the sulphuric acid has beenadded. Provided the total iron concentration is kept below the preferredlimit given above (8 grams per liter), this oxidation takes place quiterapidly; and after the orebody h as been warmed somewhat by the chemicalreaction involved in dissolution of the oxidized minerals by thesulphuric acid, the rate of the oxidation reaction increasesconsiderably.

The critical nature of proper control over iron concentration and pI-I,as these factors affect oxidation of ferrous iron to the ferric formupon exposure to the air, and as they affect precipitation of excessferrie iron, is evident from Figs.

l2 to 4. Reference first isrmade to Fig. 2, which is a series of charts(A to G) showing how variations in the total iron concentration in anaqueous solution affects the way in which oxidation of ferrous iron tothe ferrie form takes place (both as to the rate of such oxidation andthe extent `to which it occurs), and: how it further affects the extentto which iron that has been oxidized to the ferric form is precipitatedas basic ferrie sulphate. Fig. 2 is based on a series of tests made onaqueous solutions initially containing iron only in the form of ferroussulphate in the various amounts indicated, and which were each exposedquietly to the air for a number of weeks to oxidize the ferrous sulphateto ferrie sulphate. In each of these tests the pH of the solution wasadjusted at the` start to 2.0 by the addition of the proper amount ofsulphuric acid. The solid curve in each case shows the amount of ferrieiron in solution at any given time after the start of the test. Thedashed curve shows the total amount of ferrie iron present both insolution and in the red precipitate of basic ferric sulphate whichformed after the test had been under way for a while. The` solidstraight horizontal line at the topy of each ofV charts A to E indicatesthe total amount of iron present, both ferrous and ferric, and is drawnon the basis of the amount of iron present in the solution at the startof the test. It marks the maximum possible limitof ferrie iron thatcould be formed by oxidation. This straight horizontal line is omittedfrom charts F and G because the positions it should occupy are outsidethe range of the drawing. The vertical distance between the solid curveand the dashed, curve on any given time ordinate-indicates the amount ingrams per liter of ferrie iron that has at that time become precipitatedas basic ferrie sulphate. The vertical distance between the dashed curveand the horizontal straight line at the top of charts A to E on anygiven time ordinate indicates the amount of ferrous iron then still un-4oxidized.

Charts A to E show that in a period of 8 weeks 6' iron` up to about 15`grams. per liter becomes almost completely` oxidized to the ferrie form,part being precipitated as. basic ferric sulphate and part remaining insolution. As shown in charts F and G, however, higherconcentrations ofiron result in only incomplete oxidation to the ferrie form, even aftera period of 12 weeks exposure to air. (Charts F and G are based on testsin which the ratio of surface-to-volume of the solution during oxidationwas approximately half as great as in the tests on which charts A to Eare based. These two tests, however, were conducted for a longer periodof time to com,- pensate for thisdifference, and were carried out longenough to determine about the maximum extent to which the iron wouldoxidize.) Other tests have shown that at still higher ironconcentrations virtually none of the iron becomes oxidized. For example,with a solution containing about 43 grams per liter of iron, initiallyas ferrous sulphate, at a pI-I of 2, a total amount of less than 1 gramper liter of ferrie iron was formed after 12 weeks exposure of thesolution to air, and the curve drawn by plotting amount ferric ironformed against time of oxidation was substantially horizontal from theperiod of 8 weeks on, indicating that oxidation had proceeded to themaximum extent to which it would go. Itis evident from Fig. 2 that ifthe iron concentration in the solution exceeds about 15 grams per liter,it cannot be completely oxidized to the ferrie form by mere exposure toair; and that if the iron concentration exceeds about 25 grams perliter, the amount of iron that will be oxidized to the ferrie form isnot sufficient to be useful in leaching operations.

Other facts of significance are also evident from Fig. 2. The charts ofthis figure demonstrate that at a pH` of 2 and at iron concentrations upto about 10 grams per liter, most of the ferrie iron formed by oxidationremains in solution (less than 2 grams per liter being precipitated asbasic ferrie sulphate). At higher iron concentrations, however, aconsiderably larger amount of basic ferrie sulphate precipitates. Thishas an important bearing on the use of the solution for leaching inplace, because any substantial precipitation of basic ferrie sulphateduring leaching is undesirable on account of the precipitate causingplugging of the solution channels through the ore. For this reason, thesolution delivered to the ore should contain not more than 10 grams perliter of iron, and preferably notmore than about 8 grams per liter, eventhough it is possible substantially completely to oxidize the ferrousiron in solutions containing a larger total amount of iron than this. Itis pos.- sible, and in some cases is desirable, :in order to minimizethe amount of iron precipitated in the ore column during leaching, touse solutions containing, for example, only 4 grams per liter of ironaltogether.

The problem of minimizing the precipitation of basic ferric sulphateA inthe solution channels through the ore being leached of course does notaffect oxidation of used leach solution to condition it for reuse inleaching, and accordingly the effluent solution from the cementationlaunder may contain a larger total amount of iron (mostly in the ferrousform) than would be desirable in actual leaching. In fact, as pointedoutabove, the cementation operation introduces an excess of ferrous ironinto the solution. If the cementation eluent contains as much as about15 grams per liter, chart E (Fig. 2) shows that virtually all of thisiron will be oxidized to the ferric form, and that the excess over aboutgrams per liter will be precipitated as basic ferric sulphate if asufficient time for oxidation is allowed. The oxidized solution,containing not much if any more than 10 grams per liter of iron, almostall in the ferric condition, can then be used satisfactorily forleaching. If the solution from the cementation launders contains as muchas 25 grams per liter of iron altogether, it can by a sulciently longexposure to the air be oxidized enough to yield a useful concentrationof ferrie iron (as shown by chart F, Fig. 2). Accordingly the efuentfrom the cementation launder may sometimes contain up to 25 grams perliter total iron. Such high iron concentrations are not desirable,however, because the proportion of iron that becomes oxidized is rathersmall, and the excess ferrous iron at best serves no useful purpose inleaching and at worst may contribute to undesirable plugging of solutionchannels through the orebody.

Still another factor of significance in leaching in place operationsthat is apparent from Fig. 2 is the way in which iron concentration inthe solution affects the rate at which oxidation to the ferrie formtakes place. With total iron concentrations up to about 8 grams perliter, most of it (about 75% or more) is oxidized to the ferrie form bymere exposure to the air after a period of only three weeks, and afterfour weeks at a concentration of 10 grams per liter. At higherconcentrations, however, a considerably longer time of exposure isrequired in order to convert most of the ferrous iron to the ferrie Aform. At a concentration of about grams per liter, for example, five tosix weeks is required to convert 75% of the ferrous iron to the ferricform, and at still higher concentrations it is impossible to oxidizethis proportion of the iron. It is not generally economicto maintain thevery large reservoir of solution that is necessary to allow for aprolonged time of oxidation. In many places, especially in arid regionswhere low-grade porphyry copper orebodies amenable to leaching in placeare often found, it may be impractical and uneconomic to have more thanthree to four weeks supply of solution on hand. For this additionalreason it is undesirable to have more than 15 grams per liter of iron insolution, and it is preferable to keep the iron concentration even inthe effluent from the cementation launders at a substantially lowervalue than this.

Fig. 3 supplements what has been said above in connection with Fig. 2.Fig. 3 shows the relation between the total amount of iron in thesolution at the start of oxidation (the pH of the solution having beenadjusted to 2.0), and the amount of ferrie iron formed after quiescentexposure of the solution to the air for periods of eight and threeweeks. It is seen from the two upper curves of this ligure that at totaliron concentrations up to 10 and even up to 15 grams per liter(initially all present as ferrous sulphate), the amount of iron oxidizedto the ferric form in a period of eight weeks closely approaches themaximum possible (indicated by the straight diagonal line markedTheoretical Ferric Iron Limit). That is, the eiiiciency of oxidation isvery nearly 100% in a period of eight weeks. At vhigher concentrations,however, the eiciency of the oxidation drops off sharply, becomingneggligible at concentrations of about 30 grams per liter. When theperiod of quiescent oxidation is three weeks (as shown by thetwo lowercurves of Fig. 3), the sharp decline from-near100% eciency of oxidationbegins to occur at about 8 grams per liter total iron, and is virtuallyzero at 25 grams per liter. While the longest period oi time for whichFig. 3 shows the efficiency of oxidation is Vbut eight weeks, other datahas shown that at high iron concentrations (30 grams per liter and more)even periods of oxidation much longer than eight weeks result innegligible oxidation of ferrous iron to ferrie.

The data presented in Figs. 2 andV 3 are based on effecting oxidation bysimple exposure of a quiet body of solution to the air. The rate atwhich oxidation takes place can be very greatly accelerated by aeratingthe solution, as by spraying it into the air, or by bubbling air throughit. For example, substantially complete'oxidation of a solutioncontaining about grams per liter of iron can be brought about in aperiod of ve days by bubbling air continuously through the solution,whereas about seven weeks are required when the solution is simplyexposed as a quiescent body to the atmosphere. Such aeration of thesolution, however, required large amounts of power in order to treat thelarge amounts of solution required for a leaching in place operation,and it is ordinarily more economic to provide the larger reservoir ofsolution needed to permit oxidation to take place by simple exposure ofa quiescent body to the air, than to use a smaller volume and gain morerapid oxidation by powerconsuming aeration methods. The invention,however, contemplates oxidation'oi the solution in either manner.

An interesting observation made in connection with tests on the bubblingof air through ferrous iron solutions is that prolonged aeration in thisfashion of a solution containing the rather high iron concentration ofabout 30 grams per liter results in practically no oxidation of the ironin that length of time which is sufficient to effect very considerableoxidation of the iron in a solution containing only 8 grams per liter.This result confirms the showing made above that a low ironconcentration, less than 25 grams per liter, is necessary in order foroxidation by air to take place.

What has just been said.I is indicative of the fact that a lowoxidizable iron concentrations the rate of oxidation of a quiescent bodydepends directly on the area-to-volume ratio of the body during theexposure to the air. For example, the time required for substantiallycomplete oxidation of a solution initially containing 4 grams per literof ferrous iron at a pH of 2.9, at different ratios ofsurface-to-volume, is indicated in the following table (thesurface-to-volume ratio is determined by dividing the area of the bodyin units of length sqxaed by its Volume in the same units of length cu eSurface-to- Time for Substanhally Vltlige Complete Oxidation 55 l Week.ll 4 Weeks. 03 Incomplete at 8 Weeks.

aesaess 9`. for exposing it to the oxygenof. the air for purposes ofoxidation. Suche pond should be as.

broad in surface area andas shallow as is practical in order to make thevolume-to-surface ratio.

large and so to` obtain a high rate. of. oxidation of ferrous irontothe-ferric form.

The temperature of the solution is another fac-. tor exerting aconsiderable influence on tharate. However, it proceeds rapidly enoughat ordinaryout-door tem-A peratures so that the solution will oxidizeeifec. tively even in an open` pondin the winter-time..

at which oxidation proceeds.

(provided only thepond does not freeze. over.

At summer temperatures of 70. to 80 F., oxida..`

tion is quite rapid, as indicated in. Figs. 2` to 4, if the otherinfluencing factors also areproperly controlled.

In addition to controlling the concentration of, iron in the solution,which has been discussed rather fully above' in connection with Figs. 2and` 3, control over the pH of the solution alsois or very realimportance, as is apparent ironia confV in the precipitate of basicferrie sulphate that` formed. The horizontal straight line at the` topof` each chart shows the, total amount` of iron, both ferrous andferrie, present in the solution (4 grams per liter in` each case), andis, based on the amount introduced into the solution at the. start of.the test. 'I-he vertical distance between, the solid curve and thedashed curve` on any time. ordinate gives the amount offerricironllrecipi: tated as basic ferrie sulphate at that time. Thevertical distance between the dashed curve and the horizontal straightline. at the top of thechart on. any time ordinate. shows the amountcfg-fer-V rous iron then still unoxidized.

Charts H and I of Fig. 2 show that at low. pH values (corresponding. tofairly high acid con-. centrations), oxidation proceeds quite rapidly,and that substantially all of the ferrie iron re.- mains in solution(none precipitated at a pH of 1.3, and the small amount thatprecipitated at a pH of 0.8 redissolved'). At pH values of- 1.75 andabove, basic ferrie sulphate is precipitated, and the amountprecipitated increases with increasing pH. This phenomenon is ofimportance in two respects. In the rst place, it is apparent that toohigh a pH value must be avoid' ed during the leaching operation itself,or basic ferrie sulphate will be precipitated in the ore column and willplug the solution channels through it. On the other hand, too low a pHvalue must be avoided during oxidation of the ferrous iron to the ferrieform, or excess iron not wanted in the leach solution will not beprecipitated. A balance of these two opposing factors leads to adetermination that a pH of about 2 is optimum for the solutioncollecting at the bottom of the ore column and carried through thecementation launders to the pond in which oxidation takes place. Exactcontrol of the pH at precisely 2.0 is not essential, but the limits arequite close: the pH should not be much less than 1.75 or more than 2.25between the point where hilf 1'0; the solution is collected atl theVbottom of` the orebody andwhere itis oxidized by exposure to the` air intheK pond.

Another factor of interest that isapparent` from Fig. 2 is. that the`rate of oxidation decreases somewhatwith.increasingpH values above about1.25. From this value upto a pH of about 2 oxidation` isf virtually.completed. within three.

weeks, whereas. ata pH of 2.2 about four weeks arerequired., ata pH- of2.6 iive weeks are neces-` sary, and at a pH of.Y 3.3 six weeks areneeded. A pH verylmuch above.2.is forthis further reason undesirablewhenf the solution is introduced into the pond for oxidation. 0n theother hand, the

l pI-Imustnot be very much below 2, or as pointed out above, `excessferriciron will` not be precipi.

tated.

An interesting observation.. in connection` with the effect ofl pH on.the rate of oxidation has also. been made i-nI connection with strongsolutions` containing 30 grams` per liter or more of iron.

Someoxidation of a solution containing this much` iron does take placeat a pH` of. 2, as shown by4 chart G of Fig. 2. However, atpHs of 12.5and 3,

1 solutions of about this same iron concentration are almost completelyunoxidized after ten weeksH exposurelto the air, the total ferric ironpresent then being less than 2 grams perh liter..` at a `pI-I` of 2.5and only 1.5 grams per liter at a pH of` 3..

Virtually all of the ferrie iron formed atsuch pH values is found in theprecipitated basic ferrous sulphate.

Chart H of- Fig. 2, in comparison with charts- I and J, indicates thatat pH values below about` 1.25 the rate at whichoxidation of the ironpro'- ceeds is less than in the pH rang-e from 1.25 to-` 2.0. Even atlow pH `values (below 11.25), how-- ever, the rate of oxidationr israpid` enough, especially when the solution is warm, so that ferroussulphate can effectively be oxidized even when the solution is in itsmost acidfcondition (directly` after adding sulphuric acid from thestorage ves'- sel l5 of- Fig. l). This circumstance makes it entirelyfeasible to by-pass some of the ferrous y the ground by the exothermicreaction between the sulphuric acid and the oxidized copper min` eralshas the effect of accelerating such oxidation.

As previously stated, the pH of the solution coll A lected after it haspercolated through the orebody should be close to 2, but thisdoes notmean that the pH of the solution as it is delivered to the orebody isnearly so high as this. In fact, it must be lower than 2- toallow forthe consumption of V acidv that takes place in dissolving oxidizedcopper minerals from the orebody and still avoid having it increase toabove 2. The amount of sulphuric acid introduced into the solution justbefore delivering it to the orebody depends entirely onV the depth ofthe orebody, the amount of oxidized copper minerals and otheracid-consuming minerals present therein, etc., and enough acid must beincorporated at the beginning so that the solution collecting at thebottom of the ore column has a pH not lower than about 2. This mayrequire wid CQICCFWOHS ranging from 3- or 4 grams" pei' liter to 15 or20, or elven more, grams per liter (4 grains` per literl-IgSCy.vcorresponding to a pH of about 1.7, and 2O grams per liter H2SO4corresponds approximately to a pH of 1). Throughout most of the heightof the orebody, the solution percolating through it will accordinglyhave a pH lower than 2. However, toward the bottom of the ore column thepH will approach a value of 2 (due to acid consumption in dissolvingoxidized minerals, in oxidizing ferrous sulphate, etc.) and should beneither much higher nor much lower than this value. in order, on :theone hand, to insure against precipitation of much basic ferric sulphatein the solution channels in the orebody, and,- on the other hand, sothat the solution will be in satisfactory conditionl tV enable excessiron to be precipitated when the solution is oxidized (after cementingout theY copper) to condition it for reuse in leachlng.

From the time the solution is collected atrth bottomV of the orebodyuntil sulphuric acid is added to it just prior to re-delivering it tothe orebody, its pH will not be very different from 2, and Figs. 2 and 3show that at this pH value someV basic ferrie sulphate is likely toprecipitate almost any place, including inside the pipes, pumps andother apparatus'in which the solution is handled. This apparatus,therefore, should be designed so that it can readily be cleaned withoutinterfering seriously withthe leaching opera? tions.

. I claim:

Y1. The method of leaching sulphidic copper ore in place which comprisesdelivering a dilute aqueous solution of sulphuric acid and ferricsulphate to the ore, and collecting the solution after it has passedthrough the ore, characterized in that the total amount of iron presentin the solution delivered to the ore is less than grams per liter andthe amount of sulphuric acid therein is such that the pH of the solutionas it is collected after passing through the ore is not` greater thanabout 2. l

2. The method of leaching sulphidic copper ore in place which comprisesdelivering a dilute aqueous solution of sulphuric acid and ferriesulphate to the ore, and collecting the solution after it has passedthrough the ore, characterized in that plugging of the solution channelsin the ore by precipitation therein of basic ferric sulphate isprevented by limiting the amount of iron in the solution delivered tothe ore to a maximum of about 8 grams per liter and by adding to thesolution delivered to the ore sufficient sulphuric acid so that the pHof the solution collected after passing through the ore is about 2.

3. The cyclic method of leaching sulphidic copper ore in place whichcomprises delivering a dilute aqueous solution of sulphuric acid andferric sulphate to the ore, collecting the solution after it has passedthrough the ore, separating copper from the collected solution,thereafter oxidizing the ferrous iron in the solution to the ferricform, and then re-delivering the solution to the ore, characterized inthat the total amount of iron in the solution is reduced to less than 10grams per liter prior to re-delivery of the solution to the ore, andfurther characterized in that sulphuric acid is added to the solutionprior to its re-deliVery to the ore in an amount sufficient to impart tothe solution when collected after passing through the ore a pH notgreater than about 2.

4. 'I'he cyclic method of leaching sulphidic copper ore in place whichcomprises delivering a dilute aqueous solution of sulphuric acid andferthe ferrous iron in the solution to the ferrie form,

and then re-delivering the solution to the ore,V

characterized in that the ferrous iron is oxidized by introducing thesolution into a body thereof that is exposed to the oxygen of the air,the to` tal amount of iron in the solution being limitedY to less than25 grams per liter at the time of its introduction into said body andthe pH of the solution then being not less than about 2, whereby` ferrieiron in excess of the amount desired in the solution for leaching iscaused to precipitate. Y

5. The method according to claim 4, charac-V terized in that solution iswithdrawn from theV body exposed to the oxygen of the air and isredelivered to the ore only after the total iron concentration thereinhas been reduced to less than 10 grams per liter.

6. The cyclic method of leaching sulphidic copper ore in place whichcomprises delivering. a dilute aqueous solution 'of sulphuric acid andferrie sulphate to the ore, collecting the solution after it has passedthrough the ore, separating copper from the collected solution,thereafter oxidizing the ferrous iron in the solution to the ferricform, and then re-delivering the solution to the ore, characterized inthat the ferrous iron is oxidized by introducing the solution into ashallow pond of large surfacearea relative to its volume, the totalamount of 'iron in the solution being limited to less than 15 grams perliter at the time of its introduction into said pond and the pH of thesolution then being about 2, whereby ferric iron in excess of the amountdesired in the solution for leaching is caused to precipitate, andwithdrawing solution containing less than about 8 grams per liter ofiron, substantially all of which is in solution in the ferric form, fromsaid pond, and delivering such withdrawn solution to 'the ore.

7. In a cyclic method for leaching sulphidic copper ore in place,involving delivering a dilute solution of sulphuric acid and ferricsulphate to the ore and collecting the solution after it has passedthrough the ore, the steps which comprise cementing copper from thecollected solution with metallic iron, whereby ferrous sulphate isintroduced into the solution, withdrawing the solution from thecementation operation at a pH not less than about 2 and while the totalamount of iron it contains is less than 15 lgrams per liter, exposingthe withdrawn solution to the oxygen of the air for a suilcient periodof time to oxidize the ferrous sulphate therein to ferric sulphate andto precipitate suicient iron to reduce the total amount thereof insolution to less than l0 grams per liter, and utilizing the resultingoxidized solution in further leaching of the ore.

8. In a cyclic method for leaching sulphidic copper ore in place,involving delivering a dilute solution of sulphuric acid and ferricsulphate to the ore and collecting the solution after it has passedthrough the ore, the steps which comprise cementing copper from thecollected solution with metallic iron, whereby ferrous sulphate isintroduced into the solution, withdrawing the solution from thecementation operation at a pH not less than about 2 and while the totalamount of iron it contains is less than l5 grams per liter, introducingthethus-withdrawn solution into a pond of large surface area relative toits volume, wherein the solution is exposed to the oxygen 0i illgairarid its fermes iwnonteni is .uxiflizsadn 13 to the ferric form andexcess iron is precipitated from solution, and withdrawing solutioncontaining less than 10 grams per liter of iron from said pond for usein further leaching of the ore.

9. In a process of the character described, involving treating asolution containing ferrous iron to oxidize the iron to the ferric formand to precipitate excess iron from the solution, the improvement whichcomprises introducing the solution into a container, exposing thesolution in the container to the oxygen of the air, and keeping it thusexposed for a period of at least several days, characterized in that thesolution contains less than 25 grams per liter of iron and is acidiedwith sulphuric acid to a pH not less than about 2 at the time of itsintroduction into said container, whereby a large part of the iron isoxidized to the ferrie form and a part of the ferrie iron isprecipitated. I

10. In a process of the character described, involving treating asolution containing ferrous sulphate to oxidize the iron to the ferrieform and to precipitate excess iron from the solution, the improvementwhich comprises introducing the solution while it contains less than 15grams per liter of iron and is acidied with sulphuric acid to a pH ofabout 2 into a pond having a large surface arearelative to its volume,and exposing the solution in the pond to the air for a period of severalweeks, whereby substantially all of the iron is oxidized to the ferrieform and the amount of iron in solution is reduced by precipitation ofbasic ferrie sulphate to less than about 10 grams per liter.

11. The cyclic method of leaching sulphidic copper ore in place whichcomprises delivering a dilute aqueous solution of sulphuric acid,ferrous sulphate and ferrie sulphate to the ore, collecting the solutionafter it has passed through the ore, cementing copper from the collectedsolution,

delivering a portion of the solution containing iron mostly in the formof ferrous sulphate from the cementation operation to a pond wherein itis exposed to the air, the cementation of iron in such solution beingless than l5 grams per liter and the pH of the solution being about 2,whereby the ferrous iron in the solution is oxidized to the ferric form,withdrawing solution containing ferrie sulphate from the pond and mixingit with the remaining portion of solution containing ferrous sulphatefrom the cementation launder, adding sulphuric acid to said mixedsolutions, and re-delivering the mixed solutions to the ore, the amountof sulphuric acid added to the mixed solutions being suicient so thatthe pI-I thereof after draining through the ore is not greater thanabout 2.

WALTER GIFFORD SCOTT.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PA'I'EN'I'l Number Name Date 1,021,768 Gahl Apr. 2, 19121,451,734 Irving Apr.. 17, 1923 1,837,286 Oppenheim Dec. 22, 1931 OTHERREFERENCES

3. THE CYCLIC METHOD OF LEACHING SULPHIDIC COPPER ORE IN PLACE WHICHCOMPRISES DELIVERING A DILUTE AQUEOUS SOLUTION OF SULPHURIC ACID ANDFERRIC SULPHATE TO THE ORE, COLLECTING THE SOLUTION AFTER IT HAS PASSEDTHROUGH THE ORE, SEPARATING COPPER FROM THE COLLECTED SOLUTION,THEREAFTER OXIDIZING THE FERROUS IRON IN THE SOLUTION TO THE FERRICFORM, AND THEN RE-DELIVERING THE SOLUTION TO THE ORE, CHARACTERIZED INTHAT THE TOTAL AMOUNT OF IRON IN THE SOLUTION IS REDUCED TO LESS THAN 10GRAMS PER LITER PRIOR TO RE-DELIVERY OF THE SOLUTION TO THE ORE, ANDFURTHER CHARACTERIZED IN THAT SULPHURIC ACID IS ADDED TO THE SOLUTIONPRIOR TO ITS RE-DELIVERY TO THE ORE IN AN AMOUNT SUFFICIENT TO IMPART TOTHE SOLUTION WHEN COLLECTED AFTER PASSING THROUGH THE ORE A PH NOTGREATER THAN ABOUT 2.